1. Technical Field of the Invention
The present invention relates generally to electrical amplification, and more particularly to feed forward amplification.
2. Description of the Related Art
A feed-forward amplifier system was proposed by Harold S. Black in the 1920""s as illustrated by his United States patent, U.S. Pat. No. 1,686,792, which is hereby incorporated by reference for all purposes. The feed-forward amplifier system design was intended to reduce amplifier distortion, e.g., inter-modulation distortion (IMD) of carrier signals (carriers) to thereby suppress cross-talk between multiple carriers. Since then various feed-forward amplifier system designs have been proposed for reducing amplifier distortion and noise. A number of examples of the various feed-forward amplifier designs are provided in U.S. Pat. No. 5,051,704 issued to Chapman et al., hereby incorporated by reference for all purposes. The feed-forward amplifier system is now well established and particularly useful in wideband and multi-carrier systems.
One particular example of a feed-forward amplifier system used for educing distortion and noise is provided in FIG. 1. There is shown a feed-forward amplifier system having two different carrier signals, carrier signal 101 and carrier signal 102 having two different frequencies that are input into to node 110 where they produce combined signal 103. Combined signal 103 is provided through node 111 to an amplifier 112 and an adjuster 113. The output of amplifier 112 is provided through node 115 to a delay element 117 and to node 116. The output of adjuster 113 is input to delay element 114 and output therefrom to node 116. Node 111, amplifier 112, node 115, node 116, adjuster 113, and delay element 114 make up a signal cancellation loop 140. As such, ideally the output of the signal cancellation loop 140 from node 116 has had the signal of the combined carrier signals, signal 103, removed and contains only the distortion and noise produced as a result of imperfect amplification and inter-modulation products of amplifier 112. However, in some applications, for example wideband and multi-carrier amplifier systems, the carrier signals are not completely cancelled in the signal cancellation loop 140, because of non-ideal frequency response (and matching) of the amplifier 112 (and other passive components).
The output of node 115 is input to delay element 117 and the output of delay element 117 is input to node 120. The output of node 116 is input to adjuster 118. The output of adjuster 118 is input to amplifier 119 which amplifies the adjusted distortion and noise component, introduced to the signal by amplifier 112. The output of amplifier 119 is input to node 120. Node 115, node 116, adjuster 118, amplifier 119, delay element 117, and node 120 make up distortion cancellation loop 150 which operates to remove the distortion and noise component from the output signal of amplifier 112 to thereby produce amplified signal 104 reasonably free of noise and distortion introduced by amplifier 112.
In operation, the feed-forward amplifier system of FIG. 1 operates to remove the distortion, e.g., IMD, and noise that would normally occur as a result of the non-ideal characteristics and inter-modulation products of amplifier 112. First, the distortion and noise generated by amplifier 112 is isolated in the signal cancellation loop by, for example, subtracting the source signal 103 from the amplified output of amplifier 112. This signal is output from node 116 and is often referred to as the error signal. Next, the error signal is adjusted by adjuster 118, amplified by amplifier 119, and then, for example, subtracted at node 120 from the delayed (via delay element 117) amplified output signal of amplifier 112 which contains distortion and noise introduced by amplifier 112. As a result, the signal 104 output from the feed-forward amplifier system is intended to have reduced distortion and noise.
However, for some applications amplifier designs are required to be very robust to achieve the distortion and noise reduction required. This is particularly true for wideband and multi-carrier applications such as cellular telephone communications. The known feed-forward amplifier system described above suffers from poor carrier cancellation in wide-band adaptive feed-forward amplifiers for multi-carrier operation. There are several ways to attempt to overcome this problem. First, an unnecessarily large error amplifier can be used to compensate for the poor carrier cancellation. The problem with this approach is that it leads to poor system efficiency. Another approach to alleviate this problem is to specify the required performance of active and passive components from manufacturers to fit within tight phase and gain flatness tolerances and that have very good matching. This approach, however, increases the unit price of the components and has limitations in terms of what can be achieved in practice. Finally, complex and more elaborate digital control circuitry can be used in order to provide high phase and gain control accuracy and resolution, but this approach also leads to increased costs with only marginal performance improvements.
The present invention is directed to improving the quality of the signal output from a feed-forward amplifier system and/or allowing for reduction of the distortion and noise that results from the electrical characteristics of the amplifiers used in a feed-forward amplifier system over wide bandwidths. The present invention uses multiple signal cancellation loops in a feed-forward amplifier system. The double carrier cancellation method and apparatus disclosed herein improve carrier cancellation levels and at the same time avoid the disadvantages described above. In addition, the double carrier cancellation method and apparatus leads to a reduction in the size of the error amplifier needed in the distortion cancellation loop thus leading to improved system efficiency.
According to one variation of the invention, two signal cancellation loops of a two carrier multi-carrier feed-forward amplifier system are provided, wherein each loop shares the same amplifier and each loop has a separate adjuster. The separate adjusters can be used to adjust different portions of a frequency band.
Other features and advantages of the invention will become apparent through the following description, the figures, and the claims.